Long products, method of thermo-chemical treatment and apparatus

ABSTRACT

The invention provides long products with stiffness controlled by the cross sectional shape, method of their thermo-chemical treatment, and apparatus for forming the final shape of the long products and thermo-chemical treatment.

This patent application claims filing date of Provisional Patent No.60/730,653 filed on Oct. 28, 2005.

A. Field of the Invention

Present invention is related to the field of long products and theirthermo-chemical treatment, and more specifically to carburizing,nitriding, carbonitriding, boriding, and other similar processes ofsurface modification of continuous metal parts, such as wire, strip,sheet, and other long products.

B. BACKGROUND OF THE INVENTION

Long products, such as wire, strip, sheet, have dimension in onedirection significantly exceeding dimensions in the other twodirections. In many applications, stiffness and strength are criticalproperties of long products. A number of ways to control theseproperties have been developed, including controlling shape of thenormal cross section to control stiffness and surface thermo-chemicaltreatment, specifically, strip carburizing and nitriding. Meanwhile,other methods of thermo-chemical surface treatment, such as,carbo-nitriding, and boriding have been widely used in processing ofvarious metal parts. The main advantage provided by surfacethermo-chemical treatment processes is related to their ability toproduce a high hardness surface layer preserving viscous properties ofthe core material. In thermo-chemical surface treatment, a Mart isheated in a special environment, often referred to as an active mediaenvironment, containing atoms of carbon, nitrogen, their mix, atoms ofboron, or other elements, to the temperature, at which diffusionprocesses accelerate to ensure acceptable process efficiency. Afterachieving diffusion of these atoms to a desirable depth, part istypically quenched and tempered.

Nitrding, carbo-nitriding, and boriding are typically performed in aliquid or gaseous environment, and carburizing can also be done in asolid medium. The process can be accomplished as a batch process or as acontinuous process. While numerous technical solutions have beendeveloped for batch process, continuous surface thermo-chemicaltreatment processes use typical approaches and equipment utilized forthermo-chemical processing of individual parts with comparabledimensions in all directions.

As an example, U.S. Pat. Nos. 5,192,485 and 6,074,493 teach a method ofstrip carburizing in a gaseous environment. They provide a furnace forcontinuous carburizing, in which a low carbon steel strip is heated to atemperature between temperature of pearlite transformation and 980 C.Similarly, U.S. Pat. No. 5,653,824 provides a strip carburizing methodfor continuous carburizing of a strip material by using a furnacethrough which strip is moving in a carburizing atmosphere. This furnacehas a typical design for continuously heating strip with a quenchingzone at the furnace exit and means to feed said strip in the furnace andaccumulate strip on a take-up spool.

The main disadvantage of these methods is related to a relatively longprocessing time and expensive and complex equipment

U.S. Pat. No. 5,798,002 teaches a method of producing a surface layerselected from the group consisting of carbide containing surface layerand a carbon solid solution containing surface layer on a substrateselected from the group consisting of a metal and a metal containing analloying element. The method comprised the following steps: providing abath composed of a cold carbon-containing liquid active medium atambient temperature; introducing heating means into the cold liquidactive medium; immersing a substrate into the cold liquid active medium;heating the substrate directly by the heating means inside the liquidactive medium to a corresponding processing temperature until the layerof a desired chemical composition and thickness is formed on thesubstrate. However, this method was developed primarily in applicationto processing individual parts, and not to long products, such as sheetor strip.

The present invention provides a solution eliminating thesedisadvantages of the prior art.

C. SUMMARY OF THE INVENTION

Present invention provides long products with desirable combination ofstiffness, surface strength and ductility of the core, method of theirsurface thermo-chemical treatment allowing their efficient processing,and apparatus for its realization.

In one aspect, present invention provides long products with an outsidelayer having strength and corrosion resistance properties increased bythermo-chemical modification techniques, such as carburizing, nitriding,carbo-nitriding, and boriding. Stiffness of said long products iscontrolled by the shape and dimensions of cross section normal to theirlong axis. The cross sectional shape is formed by using roll forming orprofiled die drawing operations during or right after theirthermo-chemical treatment. Torsion operation can also be used to achievefinal shape of the cross section before or after their surfacethermo-chemical treatment.

In another aspect, present invention provides a method ofthermo-chemical treatment comprising the following steps:

-   -   surface cleaning the processed material    -   placing material in an active media environment in a gaseous,        liquid, solid form or in a mixture of these substances, i.e.        gaseous-liquid, liquid-solid, gaseous-solid, or        gaseous-liquid-solid form    -   locally heating the material at least in one region to a        predetermined temperature    -   forming the final shape of cross section    -   moving material through the heater and shape forming tool        processing all of the material or only selected regions    -   cooling the material with a predetermined rate

Acid solutions, such as hydrochloric acid, nitric acid, sulfuric acid orany other cleaning solutions and methods, including mechanical methods,are used to clean surface of processed material.

Material is exposed to active media environment. For example,carburizing is accomplished by placing material in a container with acarbonous gas, liquid, solid, or a mixture of these substances.

Material is locally heated to a predetermined temperature at least inone location. Heating temperature and holding time at that temperaturedepend on desirable depth of the processed layer.

In one implementation, material is heated to typical carburizing,nitriding, carbo-nitriding, or boriding temperatures that fall in atemperature interval from 560 to 950 C depending on type of surfacetreatment process and active media used.

In another implementation, material is heated to up to 500 C abovetypical temperatures, i.e. to temperature of up to 1450 C.

In still another implementation, multiple material areas are heated todesirable processing temperate.

Final material dimensions and shape are ensured by using metal formingoperations.

In one implementation, material is formed to the final shape duringsurface thermo-chemical processing.

In another implementation, material is formed to the final shape aftersurface thermo-chemical processing is complete, but material is stillhot, i.e. at temperature resulting in good material formability.

In still another implementation, meal is formed to the final shape aftersurface thermo-chemical processing is complete at ambient temperature.

Material is being fed into heating zone and spooled on a take up spoolunder controlled tension.

In one implementation, tension is maintained below material yieldstrength to avoid shape changing.

In another implementation, tension exceeds material yield strength toachieve desirable shape changing, for example, wire diameter reduction.

Cooling after surface thermo-chemical processing is done with a rateproviding desirable final properties.

In one implementation, cooling is done with a rate exceeding criticalquenching rate to a predetermined temperature at which material istempered and then cooled to the ambient temperature.

In another implementation, cooling is done with a rate below criticalquenching rate continuously to the ambient temperature.

In still another implementation, cooling is done according to a regimeincluding cycles of heating and cooling.

In still another aspect, present invention provides an apparatus forthermo-chemical modification of long products. Said apparatus comprisedthe following blocks: means of feeding processed material into thecontainer with active media, heating means, means of supportingprocessed material inside the heating zone, shape forming means, coolingmeans, means of applying pre-determined tension on the strip, and meansof taking processed material out of the processing zone and storing itin a desirable way.

In one implementation, said apparatus allows simultaneous processing ofmore than one long product.

In another implementation, heating means include direct or indirectresistance heating, induction heating, plasma hag, laser heating, or afurnace used for metal heat treating. Such heating means as inductionheating or direct resistance heating can be positioned inside or outsideof the container with active media.

In still another implementation, container with an active media can bepositioned inside another container with cooling means that can servethe purpose of keeping desirable temperature of active media and to coolprocessed material with a desirable cooling rate. In still anotherimplementation, active media is circulated through the processing zoneor agitated.

In another implementation, at least two different types of heating meansare used. In another implementation, tension applied to the processedmaterial is predetermined in such a way that said material is selfsupported.

In another implementation, means of material support include a conveyoror set of rollers.

In another implementation, shape forming means include shaped rolls,dies, or any other means of shaping material in a desirable way.

In another implementation, means of storing processed material caninclude devices for putting said material on a spool or cutting it tothe pre-determined length, for example, cutting it in short fibers.

Present invention allows processing of long products manufactured ofdifferent metals: low carbon steel, high carbon steel, stainless steeltitanium alloys, aluminum alloys, tungsten alloys, and any other metalshowing improved strength, corrosion, impact strength, or any othertargeted properties after thermo-chemical processing.

One set of characteristic features of the present inventiondifferentiating it from prior art is related to the long productssubjected to thermo-chemical processing.

In contrast to currently used long products subjected to surfacemodification processing limited to the strip, present invention broadenstypes of long products subjected to surface modification processing toother types, such as wire, cord, strip with a profiled cross section, orany other type of products with their stiffness controlled by dimensionand shape of normal cross section.

In contrast to currently used long products subjected to surfacemodification processing, present invention provides said product withincreased effective strength.

Another set of characteristic features of the present inventiondifferentiating it from prior art is related to the method ofthermo-chemical processing.

In contrast to standard methods of surface modification methods, presentinvention provides temperature regime of up to 500 C higher thantemperatures used in standard methods.

In contrast to method disclosed in U.S. Pat. No. 5,798,002 that teachesvarious types of surface modification in liquid active media, currentinvention provides solutions utilizing gaseous, solid forms of activemedia, as well as mixtures of gas-liquid, liquid-solid, and gas-solidactive media.

In contrast to prior art, in present invention, thermo-chemicalprocessing is combined with shape forming operation that increases depthof the processed layer.

In contrast to prior art, present invention allows achieving increasedratios of depth of the processed layer to the smallest transversedimension of the processed material. While the depth of the layer withmodified structure and properties in prior art is typically only a smallfraction of the part's cross sectional dimension, according to thepresent invention, it can be significantly larger reaching one in caseof fully modified cross section. For instance, in case of a smalldiameter filament or thin strip and foil, the modified layer can be allthrough the cross section.

Still another set of characteristic features of the present inventiondifferentiating it from prior art is related to the provided apparatusfor thermo-chemical treatment.

In contrast to the existing solutions, present invention provides anapparatus capable of continuously processing more than one long productsimultaneously.

In contrast to the existing solutions, means of continuously feeding theprocessed material with a predetermined tension and speed allowdirectionally grow microstructural components in the modified layer.

In contrast to the existing solutions, present a can include heatingmeans of different types can be used simultaneously.

In contrast to the existing solutions, present apparatus includes shapeforming means are provided.

In contrast to the existing solutions, present apparatus can includespecial supporting means, such as conveyor or set of rolls.

In contrast to the existing solutions, present apparatus can includeproduct storing means allowing cutting processed product to thepre-determined length.

D. BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a schematic illustration of a set up for thermo-chemicalprocessing and shape forming of a wire by using electric resistanceheating and liquid active medium;

FIG. 1B is a schematic illustration of a figure eight wire cross sectionand macrographs showing straight and twisted wire:

FIG. 2 is a schematic illustration of a set-up for thermo-chemicalprocessing and shape forming of a wire by using induction heating in anactive medium;

FIG. 3 is a schematic illustration of a set-up for thermo-chemicalprocessing using cooled tank with active media;

FIG. 4 demonstrates optical micrographs showing cross section andmicrostructure of a carburized strip with a profiled cross section;

FIG. 5 shows original grating strip and rolled and carbo-nitridedgrating strip;

FIG. 6 shows carburized wire mesh;

FIG. 7 shows carbo-nitrided woven wire mesh, and

FIG. 8 shows different grades steel wool.

E. DETAILED DESCRIPTION OF THE INVENTION

Present invention provides long products with desirable combination ofstiffness, surface strength and ductility of the core, method of theirsurface thermo-chemical treatment allowing their efficient processing,and apparatus for its realization.

Desirable stiffness properties of long products are achieved bycontrolling shape and dimensions of the normal cross section, whilestrength and ductility are controlled by utilizing their thermo-chemicaltreatment.

In one aspect, present invention provides long products with an outsidelayer having strength and corrosion resistance properties increased bythermo-chemical treatment methods, such as carburizing, nitriding,carbo-nitriding, and boriding.

In one implementation, shape of the normal cross section is produced byusing roll forming operation or drawing through profiled dies during orafter thermo-chemical treatment.

In another implementation, final shape of the normal cross section isproduced by using torsion operation around the long axis before surfacethermo-chemical processing or after it is complete.

In another aspect, present invention provides a method ofthermo-chemical treatment of long products comprising the followingsteps:

-   -   surface cleaning the processed material    -   placing material in an active media environment in a gaseous,        liquid, solid form or in a mixture of these substances, i.e.        gaseous-liquid, liquid-solid, gaseous-solid, or        gaseous-liquid-solid form    -   locally heating the material at least in one region to a        predetermined temperature    -   forming the final shape of cross section    -   moving material through the heater and shape forming tool        processing all of the material or only selected regions    -   cooling the material with a predetermined rate

Acid solutions, such as hydrochloric acid, nitric acid, sulfuric acid,or any other cleaning solutions or methods, including but not limited tomechanical de-scaling, brush rust removal, are used to clean surface ofprocessed material. The material can have shape of wire, multiple wiresarranged in a twisted bundle or in parallel, with circular, square,hexagonal, or any other shape of cross section.

Material is exposed to active media environment in a gaseous, liquid,solid form, or in a mixture of these substances, i.e. gaseous-liquid,liquid-solid, or gaseous-solid form. For example, carburizing isaccomplished by exposing material to a carbonous gas, liquid, solid, ormixture of these substances.

In one implementation, material is placed in the container with anactive media substance. For instance, in carburizing, said material isplaced into container with a carbonous gas, such as methane, carbonousliquid, such as carburizing oil, solid, such as graphite powder, ormixture of these substances. Examples of mixtures of gaseous liquid,liquid-solid, gaseous-solid, or gaseous-liquid-solid forms ofcarburizing media include, but are not limited to mixtures ofcarburizing oil with vaporized oil or methane gas, mixtures ofcarburized oil with graphite powder or carbon based grease, mixtures ofgraphite powder with methane, similar to fluidized furnace media, andmixture of methane, carburizing oil, and graphite powder, respectively.

In another implementation, active media is continuously delivered to thematerial during processing. For instance, liquid carburizing solutionsare supplied to the heated region of material as a continuous stream.Similarly, carburizing gas is supplied to the heated portion of thematerial. In a particular case, this process can be combined with awelding processes to improve mechanical properties of the weld.

The mixture of active media agents can be prepared to different ratiosof components. In the case of mixing liquid and solid components, theirratio can be from 10% to 90%, preferably in the range typical for greasecontaining corresponding active media agents, such as graphite andcarbon based grease or boron based grease. The mixture can be applied tothe material before processing. Such a coating can also be dried outbefore processing. In the case of mixed gaseous and liquid active media,ratio of components is controlled to achieve uniform materialmicrostructure and properties. For example, carburizing gas can bedelivered to the processed material submerged into the liquid, or in aform of vaporized carbonous liquid with a predetermined vapor pressure.Container capable of withstanding high pressures is used in the casewhen vapor pressure significantly exceeds ambient pressure. Such a highcarburizing pressure can facilitate carbon diffusion into processedmaterial.

Both in the case of thermo-chemical treatment of material in a containerwith a stationary active media or in case of active media continuouslysupplied to the heated region, used means of delivering active media,exposing material to it, and evacuating active media are designed toaccommodate material movement through the container. For instance, m thecase of gaseous carburizing, endo-gas, comprised 40% hydrogen, 40%nitrogen, and 20% carbon monoxide mixture is delivered to the processingzone from two opposite ends of a processing unit to reduce loss ofendo-gas.

Material is locally heated to a predetermined temperature at least inone location. Heating temperature and holding time at dig temperaturedepend on desirable depth of the processed layer.

In one implementation, material is heated to typical carburizing,nitriding, carbo-nitriding, or boriding temperatures that fall in atemperature interval from 560 to 950 C depending on type of surfacetreatment process and active media used.

Heating can be accomplished by using any kind of furnace, directelectric resistance heating, induction heating with the inductor placedin the container or positioned outside the container, laser heating, orplasma heating. In case of using induction heating, inductor can operateon more than one frequency to achieve desirable carburizing depth. Theinduction coil can be placed inside the container with active media oroutside said container. When wire fed through induction coil placedinside the container with active media, special means of avoidingcontact between processed material and said induction coil are used.These can include ceramic tube or providing sufficient tension of theprocessed material to prevent its bowing and sagging during heating.

In another implementation, material is heated to up to 500 C abovetypical temperatures, i.e. to of up to 1450 C. To avoid extensiveaustenite grain growth, heating is accomplished by means of intensiveheating, such as direct electric resistance heating, induction heatingwith the inductor placed in the container or positioned outside thecontainer, laser heating, or plasma heating.

In still another implementation, multiple material areas are heated toprocessing temperatures. This is accomplished by moving the materialthrough a set of heating means heated to predetermined temperatures.

Final material dimensions and shape are ensured by using metal formingoperations.

In one implementation, material is formed to the final shape duringmaterial thermo-chemical processing. Heated to the processingtemperature, material is shaped by using shape forming means, such asshaped rolls or dies. In case of electric resistance healing, contactelectrodes can have shape or shape forming inserts that ensure finalmaterial shape. As an example, FIG. 1A shows a set-up for carburizingand shaping material into a wire with figure eight cross section byusing grooved roller electrodes and electric resistance heating in acarbonous environment (FIG. 1B). The material moving between theelectrodes is heated by passing electric current through it and isshaped at the same time. In this specific example, material has a figureeight shape. Plastic deformation occurring during final shape formingfacilitates thermo-chemical treatment increasing penetration depth ofcarbon, nitrogen, or boron atoms. Such a shape can be further subjectedto plastic deformation, for instance, twisting. In another example,material is heated by using induction heating and then shaped by usingprofiled rolls or die (FIG. 2). In another implementation, material isformed to the final shape after thermo-chemical treatment process iscomplete, but material is still hot, i.e. at temperature resulting ingood material formability. Using profiled rolls or a die allows oneachieving desirable final shape while material is still hot withoutexposing shape forming means to high temperature and active media.

In still another implementation, material is formed to the final shapeat ambient temperature after thermo-chemical treatment process iscomplete. This operation is similar to coining and is used when highaccuracy of final dimension is required.

Material is being fed into processing zone and spooled on a take upspool under controlled tension.

In one implementation, tension is maintained below material yieldstrength to avoid shape changing. Material is fed into processingchamber by using wire or strip feeding-like equipment utilizing pairs ofrotating grooved rolls and spooled on a take-up spool. Feeding speed andspeed of winding on a take-up spool are maintained to avoid plasticdeformation, for instance reduction in material cross section size.

In another implementation, tension exceeds material yield strength toachieve desirable shape changing, for example, wire diameter reduction.If material winding speed on a take-up spool exceeds feeding speed,changes in material size will occur in areas of local heating. Forexample, in case of a circular cross section wire, this will result indie-less forming, i.e. diameter reduction without using shape formingdie. Increased material strength due to thermo-chemical treatment canfacilitate stability of material flow in die-less process. For example,in case of carburizing, low carbon wire entering heated zone has alarger cross section than that of carburized portion exiting the heatedzone. Normally in die-less drawing, such difference in wire diametersresults in localized material flow and wire break in the heated zone.However, increased strength of carburized wire can prevent such plasticflow localization making process more stable. In still anotherimplementation, wire winding is done with a controlled speed to formdesirable morphology of microstructural components formed afterthermo-chemical treatment. For instance in carburizing, slow windingspeed in combination with small cross sectional dimensions of theprocessed material can result in directional growing of cementite toachieve crystal whisker-like properties, i.e. strength close to that ofideal crystal in the range of from 8,000 to 10,000 MPa. Speed of movingmaterial through the carburizing medium can also be controlled to formhigh strength carbon-based surface deposits, such as carbon nano-tubes.In this case, catalytic elements can be used to serve as carbonnano-tube nucleation points or to facilitate their growth.

Cooling after thermo-chemical processing is done with a rate providingdesirable final properties.

In one implementation, cooling is done with a rate exceeding criticalquenching rate to a predetermined temperature at which material istempered and then cooled to the ambient temperature. To achievemartensitic structure resulting in high strength properties, material isquenched in the medium used for thermal-chemical treatment, if thismedium is a liquid, outside the heating zone. If medium used forthermal-chemical treatment does not provide cooling rate sufficientlyhigh to achieve desirable strength properties, as, for example, in caseof gaseous or solid carburizing, additional cooling means can be used,for instance by using intensively circulated gas.

In another implementation, cooling is done with a rate below criticalquenching rate continuously to the ambient temperature. It can beaccomplished in one of the local heated areas, in oil or in airdepending on the material chemical content.

In still another implementation, cooling is done according to a regimeincluding cycles of heating and cooling. In case when post quenchingannealing/tempering is to be done more than once, for example in highspeed steel, material can be thermo-cycled to achieve desirablecombination of strength and ductility.

In still another aspect, present invention provides an apparatus forthermo-chemical modification of long products. Said apparatus comprisedthe following blocks: means of feeding processed material into thecontainer with active media, heating means, means of supportingprocessed material inside the heating zone, shape forming means, coolingmeans, means of applying pre-determined tension on the strip, and meansof taking processed material out of the processing zone and storing itin a desirable way.

In one implementation, said apparatus allows simultaneous processing ofmore than one long product.

In another implementation, heating means include direct or indirectresistance heating, induction heating, plasma heating, laser heating ora furnace used for metal heat treating.

Such heating means as induction heating or direct resistance heating canbe positioned inside or outside of the container with active media.

In still another implementation, container with an active media can bepositioned inside another container with cooling means that can servethe purpose of keeping desirable temperature of active media and to coolprocessed material with a desirable cooling rate (FIG. 3).

In still another implementation, apparatus allows circulating activemedia through the processing zone or to agitate it. For example, in thecase of gaseous carburizing, gas is circulated through the processingzone keeping composition of gas constant. Similarly, in the case ofliquid carburizing, carbonous liquid is circulated through theprocessing tank to keep its composition constant.

In another implementation, at least two different types of heating meansare used.

In another implementation, tension applied to the pressed material ispredetermined in such a way that said material is self supported.

In another implementation, means of material support include a conveyoror set of rollers.

In another implementation, shape forming means include shaped rolls,dies, or any other means of shaping material in a desirable way.

In another implementation, means of storing processed material caninclude devices for putting said material on a spool or cutting it tothe pre-determined length, for example, cutting it in short fibers.

While continuous thermo-chemical treatment is a preferable method ofprocessing long products as discussed above, it also can be accomplishedas a batch processes. Material in a form of a coil, such as wire coil,strip coil, or coil of any other type of long product is placed in anactive media environment and heated by using furnace or inductionheating.

Present invention allows processing of long products manufactured ofdifferent metals: low carbon steel, high carbon steel, stainless steel,titanium alloys, aluminum alloys, tungsten alloys, and any other metalshowing improved strength, corrosion, impact strength, or any othertargeted properties after thermo-chemical processing. Depending on thematerial, surface layer in the chemically modified layer providingdesirable properties can include titanium carbide, nitride, or boridelayer or particles, aluminum nitride or boride layer or particles,tungsten carbide, nitride, or boride layer or particles, or any othertypes of hard layer or particles.

While specific processing temperatures and time depend on the material,similar to those of steel they exceed standard temperatures to ensurereduced time and incorporate fast heating. Specifically, in the case oftitanium filament, heating is accomplished to the temperature above 950C in a liquid carburizing medium, such as silicon oil. It can be used onlong products and parts produced from long products, such as dentalimplants, stents, spokes, and other similar products.

In case of aluminum wire, surface oxide film is removed before desirablesurface chemistry is achieved. For instance, a wire drawing die is usedto expose fresh surface to produce a surface aluminum nitride layer.Heating temperate is between 450 and 620 C depending on alloycomposition.

In case of tungsten long products, surface modification is accomplishedat temperatures between 1400 C and 2500 C. It can also be used forprocessing such parts, as heavy armor penetrator or other similar partsto produce hard local areas.

F. EXAMPLES OF SPECIFIC IMPLEMENTATION Example 1

Low carbon steel wire (1020 grade) with diameter of 1.5 mm was carburzedby using current process. Wire sample, 100 mm long, was clamped at theends of two copper electrodes, and submerged in a silicon oil. The wirewas heated by passing electric current through it to differenttemperatures, held at the temperature for a specified time, and thencooled down to ambient temperatures inside the processing tank or in theair. Various microstructures were achieved When wire was heated totemperature below 950 C, maximum carbon content at the surface was lessthan 1.2%, and corresponding microstructure was comprised pearlite andsecondary cementite network. Structure of the wire core did not changeand was comprised ferrite and pearlite. When wire was heated totemperature in the range from 1200 C and 1450 C, carbon concentration atthe surface as high as close to 4.5% was achieved. Correspondingmicrostructure was comprised pearlite and primary cementite when coolingrate controlled by adjusting electric current to the ambient temperaturewas slow. In case of fast cooling, structure was comprised primarycementite and martensite. Successive annealing resulted intransformation of martensite into spheroidal pearlite.

Example 2

High carbon steel wire (1090 grade) with diameter of 0.5 mm wascarburized by using current process. Wire sample was continuously fedthrough two copper electrodes submerged in a machine oil. The wire washeated by passing electric current through it to different temperatures.Wire feeding speed and take-up speed were synchronized. Variousmicrostructures were achieved similar to the stationary heated wire.When wire was heated to temperature below 950 C, surface microstructurewas comprised pearlite and secondary cementite network, while in thebulk was comprised ferrite and pearlite. When wire was heated totemperature in the range from 1200C and 1450 C, surface microstructurewas comprised pearlite and primary cementite.

Example 3

A high carbon steel wire (1080 grade) with diameter of 0.3 mm was liquidnitrided by using present method. Wire sample was continuously fedthrough a nitriding salts bath. A thin compound layer was produced aftertreatment giving wire excellent corrosion resistance properties.

Example 4

A low carbon steel strip (1020 grade) with 4×0.4 mm² cross sectiondimension was rolled between two water cooled copper disc contacts witha profiled shape. Disc contact electrodes and strip were submerged intoa machine oil tank. The strip was heated with electric current appliedto the strip, and forming pressure was applied to the discs to achievedesirable shape of the strip cross section. After processing, a wirewith a profile and microstructures shown in FIG. 4 was obtained.

Example 5

A low carbon steel grading strip with thickness of 0.3 mm was obtainedby using rolling (FIG. 5). Said grading strip was carburized in machineoil by using induction heating. The strip was completely carburizedthrough the whole cross section.

Example 6

A low carbon steel wire mesh was carbon-nitrided by usingcarbon-ntriding furnace process (FIG. 6) and oil quenched. Said wiremesh showed significant brittleness in as-quenched condition. Successivetempering in temperature interval from 400 to 650 C resulted inincreased material ductility.

Example 7

A low carbon two layer woven mesh with wire diameter of 0.25 mm (FIG. 7)was carburied by using induction heating in a graphite-machine oil mix.As-carburized material showed significant brittleness. Successivenormalizing at 850 C for 5 min. with cooling in air resulted inincreased material ductility.

Example 8

Different grades of steel wool produced out of low carbon steel FIG. 8)were carburized by using a 40% nitrogen gas, 40% hydrogen gas, and 20%carbon monoxide gas mixture and induction heating. Significant incresein strength characteristics with acceptable ductility was obtained aftertempering at 650 C.

1. Long products with an outside layer having strength and corrosionresistance properties increased by thermo-chemical modificationtechniques and stiffness controlled by the shape and dimensions of crosssection normal to their long axis formed during or right after theirthermo-chemical treatment.
 2. Method of thermo-chemical treatmentcomprising the following steps: surface cleaning placing material in anactive media environment in a gaseous, liquid, solid form or in amixture of these substances, i.e. gaseous-liquid, liquid-solid,gaseous-solid, or gaseous-liquid-solid form locally heating the materialat least in one region to a predetermined temperature forming the finalshape of cross section moving material through the heater and shapeforming tool processing all of the material or only selected regionscooling the material with a predetermined rate.
 3. Apparatus forthermo-chemical modification of a single or multiple long productscomprised means of feeding processed material into the container withactive media, heating means, means of supporting processed materialinside the heating zone, shape forming means, cooling means, means ofapplying predetermined tension on the strip, and means of takingprocessed material out of the processing zone and storing it in adesirable way.
 4. The long product of claim 1, wherein the final shapecomprised plurality of parallel members connected by regions withsubstantially smaller cross sectional area.
 5. The long product of claim1, wherein the final shape comprised plurality of parallel membersconnected by regions with substantially smaller cross sectional areahaving slots of different shape.
 6. The long product of claim 1, whereinthe final shape comprised plurality of parallel members connected byregions with substantially smaller cross sectional area twisted aroundthe long axis.
 7. The method of thermo-chemical treatment of claim 2,wherein one or multiple local areas of material in a shape of a strip,grating, wire, or multiple wires arranged in a twisted bundle or inparallel, are heated up to 1450 C.
 8. The method of thermo-chemicaltreatment of claim 2, wherein material is exposed to active mediaenvironment in a gaseous, liquid, solid form, or in a mixture of thesesubstances, i.e. gaseous-liquid, liquid-solid, solid gaseous, orsolid-liquid-gaseous form.
 9. The method of thermo-chemical treatment ofclaim 2, wherein active media is continuously delivered to the materialduring processing.
 10. The method of thermo-chemical treatment of claim2, wherein the final step of cooling with a predetermined rate includesquenching and annealing providing a combination of strength andductility properties.
 11. The method of thermo-chemical treatment ofclaim 2, wherein cooling is done with a rate below critical quenchingrate continuously to the ambient temperature by using one or multipleheating sources controlling temperature.
 12. The method ofthermo-chemical treatment of claim 2, wherein the final shape formingoperations include a combination of roll forming, wire drawing,twisting, piercing, slitting, and coining.
 13. Apparatus of claim 3,wherein heating means include direct or indirect resistance heating,induction heating plasma heating, laser heating, or a furnace. 14.Apparatus of claim 3, wherein container with an active media positionedinside another container with cooling means to keep desirabletemperature of active media and to cool processed material with adesirable cooling rate.
 15. Apparatus of claim 3, wherein active mediais being circulated through the processing zone.